Crown galls develop following cold injury to trunks on Riesling vines in the Finger Lakes region of New York. (Courtesy Thomas Burr)

Researchers are making strides in efforts to combat crown gall disease and to develop clean plants free of the pathogen that causes it, but there’s still much to be learned about how to prevent, diagnose and eliminate it in grapevines.

That said, if there’s any place that should be able to grow disease-free vineyards, it’s Washington, with its desert, sandy soils and lack of the wild grapevines that serve as reservoirs for the disease elsewhere, said Dr. Thomas Burr, Cornell University professor of enology and viticulture.

“You have a real advantage in Washington,” he told attendees at the inaugural Ravenholt Lecture Series at the Ste. Michelle Wine Estates WSU Wine Science Center in Richland in January. “If you can plant a vineyard with clean plants, you certainly are not going to eliminate the presence of crown gall forever, but if you can get a healthy vineyard started, you’re way ahead.”

The pathogen that causes crown gall disease, Agrobacterium vitis, can live silently in the vines until an injury initiates an infection, most commonly those caused by freezing temperatures or at graft unions.

The infection causes galls that crush the vascular tissue of the plant, restricting the movement of water and nutrients, and impede the vine’s ability to heal the wound. The infection also can cause necrotic lesions on roots.

The disease kills young vines that develop it at graft unions and can stress older vines, depending on the level of infection, significantly reducing vigor and yield. Once established in a vineyard, there is no chemical control.

New knowledge about A. vitis

Researchers at Cornell took cuttings from 10 different grapevines in the winter of 2014, sampled from nodes and internodes, starting at the base and working their way out through the cane.

They hypothesized that the pathogen would be at its highest level at the cane base and at nodes, Burr said.

They were wrong. It turns out the bacterium can be randomly distributed in dormant grape canes.

“It’s not only at nodes, but it’s also at internodes. It’s not only at the base, you can find it way out,” he said. “It’s not what we wanted, obviously. If you’re going to be sampling a certain block for a grower, you want it to be simple.”

That means that if a sample is positive, researchers can positively say that crown gall is present. But if it’s negative, they can’t say for certain that it’s not present somewhere else in the vine. “The more we study this bacterium, the more places we find it,” he said.

That includes in wild grapevines. In another study, researchers sampled wild grapevines in New York in 2013 and 2014 for tumorigenic strains of the pathogen, finding that roughly one-third were positive for it — 18 of 54 samples in 2013, and 12 of 41 samples in 2014.

Similar tests of wild grapevines in riparian areas in California showed 24 of 87 positive samples in 2014 and 2015, he said.

“It’s a bit shocking, but good to know that it is an important source of the pathogen in the environment,” he said.

But what researchers still don’t know, he added, is whether it can survive in other places in the environment, such as water, soil, weeds or other plants that could serve as reservoirs for it.

Finding and controlling A. vitis

Researchers have been studying this pathogen for more than 30 years but have never had a good method for detecting the bacterium that causes it. Tests to ensure plants are clean and to identify infected plants can take weeks.

However, a new test developed at Cornell University by plant pathologist Kameka Johnson would shrink that time to just three to four days.

Magnetic capture hybridization involves probing material for a specific DNA sequence to determine if the bacterium is present. For crown gall, that involves the virD2 gene in the bacterium, which is essential for infection.

“This technique can only be used if you’re looking for a specific target,” Burr said. “Because agrobacterium needs virD2 to form galls, it really was just the perfect method for detecting crown gall in grapevines.”

The assay avoids the detection of bacteria that are present in vines and do not cause crown gall, Burr said.
Researchers also see gains in biological control of crown gall.

Another strain of A. vitis, F 2/5, is a non-tumorigenic strain that originated in South Africa. Researchers discovered that applying F 2/5 along with a tumorigenic strain to inoculate the plant, crown gall would not materialize.

However, F 2/5 still causes necrosis of the roots, so researchers worked to see if they could develop a derivative of F 2/5 that was necrosis-free.

They knocked out the gene tied to causing the necrosis and found that it still maintains the ability to knock down the tumorigenic strain of A. vitis. They are working to develop the practice for commercial use.

“This is something that could be quite useful, both for treating grafts and treating dormant cuttings before rooting, two sites that could be infected with crown gall,” he said.

Advice for growers

In the meantime, growers should take steps to try to prevent crown gall in vineyards, Burr said.

These include planting certified clean material and varieties that are more tolerant of the disease — all Vitis vinifera are susceptible, but some more so than others — in sites with good air circulation and well drained soil.

Two sites in the National Clean Plant Network provide the best options for plant materials for Washington growers: Clean Plant Center Northwest and the Russell Ranch in California, which is at lower risk for the disease than other out-of-state centers but is not risk free, said Michelle Moyer,

Washington State University plant pathologist. That’s because plants that are produced in a warm location could be infected and not show symptoms until they are exposed to a freeze after being planted in Washington.

Growers should reduce likelihood of cold damage by hilling around the trunks and train with multiple trunks, Moyer said. “The galls don’t appear until after bloom — after you’ve pruned — so keep some of those suckers as an insurance policy if you’ve had a cold snap,” she said.

Burr added that growers should manage vine growth with minimal amounts of water, while keeping vines in good shape heading into winter to protect against freeze damage as much as possible.

Kevin Judkins of Inland Desert Nursery made a similar point. Some growers are pushing the limit on what they can grow where — such as Cabernet Sauvignon in too cold an area.

They need to be more selective about pairing growing sites and varieties and follow good management practices to prevent cold injury, he said. •